Low frequency acoustic and seismo-acoustic projectors find applications in underwater ocean acoustic tomography, long-range acoustic navigation and communications and deep-bottom penetration seismic profiling in the offshore oil and gas industry. Such sources may be used in Arctic under-ice acoustic far-range navigation and communications, underwater global positioning systems (RAFOS), and long-range ocean acoustic tomography and thermometry. Low-frequency underwater sound sources should be powerful and efficient.
The low frequency source can be an explosive (dynamite), or it can use more complicated technology like an air gun providing single pulses, or like vibroseis providing continuous frequency sweeps. Some acoustic sources in use for seismic applications, such as air gun, plasma (sparker) sound sources and boomers, are of the impulse type, where the transmitter emits a large non-coherent pressure pulse during a short time interval. Seismic air-gun surveys, such as those used in the exploration of oil and gas deposits underneath the ocean floor, produce loud, sharp impulses that propagate over large areas and increase noise levels substantially. Their signal is not highly controllable, either in frequencies content or repeatability. Coherent sound sources such as marine vibroseis can be much quieter and potentially less harmful for marine environments and should be used instead of air-guns in certain exploration activities. Current continuous wave type sources make use of hydraulic, pneumatic, piezo-electric or magneto-strictive drivers and different type of resonance systems to store acoustic energy and to improve impedance matching, when generating low-frequency sound waves in water. The power output of a simple acoustic source is proportional to the squares of volume velocity and frequency and needs a large vibrating area to achieve reasonable levels. As result the sound source can become unacceptably large and expensive.
One proposal is a gas filled bubble as a means of modifying the acoustic load impedance on the radiating face of a transducer, such as that shown in U.S. Pat. No. 3,219,970 to Sims. The Sims patent, however, discloses a low 3-5% efficiency.